Methods for detecting the state of charge (SOC) of battery packs in cell systems include, for instance, those proposed in Japanese Patent Application Publication Nos. 2007-220658 and 2011-150876.
Japanese Patent Application Publication No. 2007-220658 discloses a battery pack having a plurality of non-aqueous secondary batteries A as a main constituent, and in which at least one storage element B having a nonaqueous electrolyte for voltage detection is connected in series to the plurality of non-aqueous secondary batteries A. Batteries exhibiting a small change in voltage with respect to the range of SOC in which the battery is mainly used are utilized herein as the non-aqueous secondary batteries A that are the main constituent of the battery pack. By contrast, a battery exhibiting a large change in voltage with respect to the range of SOC in which the battery is mainly used is utilized herein as the storage element B for voltage detection. Japanese Patent Application Publication No. 2007-220658 proposes the feature of detecting the SOC of the battery pack of the non-aqueous secondary batteries A depending on the change in voltage of the non-storage element B.
Japanese Patent Application Publication No. 2011-150876 discloses a battery pack in which a discharge curve of first unit cells exhibits a substantially flat characteristic and the discharge curve of second unit cells exhibits a sloping characteristic, and wherein the SOC or DOD (depth of discharge) of the battery pack is detected on the basis of the terminal voltage of the second unit cells.
Thus, PTLs 1 and 2 disclose a configuration of a battery pack that has, as a main constituent, batteries A exhibiting small changes in voltage within a range of SOC set beforehand, and in which a battery B exhibiting a large change in voltage within the above range of SOC is connected in series, for capacity detection, to the battery pack. The above citations disclose the feature of estimating the SOC (state of charge) or DOD (depth of discharge) of the battery pack, having the batteries A as a main constituent, on the basis of the open circuit voltage (terminal voltage) of the battery B. These features can be built, for instance, into a charge and discharge control system for charging and discharging at appropriate timings, through estimation of the SOC of the battery pack.